1. Field of the Invention
The invention relates to synthetic polymeric resin compositions useful for thermoplastic molding and more particularly relates to a moldable blend of aromatic polycarbonates and polyurethanes.
2. Brief Description of the Prior Art
Aromatic polycarbonate resins are a well known class of synthetic polymeric resins, generally prepared by the reaction of a polyhydric phenol with a carbonate precursor; see for example U.S. Pat. No. 3,989,672. These resins, for the most part, are thermoplastic and readily molded under heat to fabricate a wide variety of articles such as automotive parts, tool housings and like structural components. The technique of molding and advantageous moldable compositions comprising the base polycarbonate in admixture with a wide variety of fillers, reinforcing agents, stabilizers, melt-viscosity modifiers, strength and impact modifiers, fire retardants, pigments and like additives are also well known. Representative of such moldable compositions are those described in the U.S. Pat. Nos. 3,431,224 and 4,481,331.
In spite of the numerous thermoplastic polycarbonate molding resin compositions known and available, there has remained a need for thermoplastic resin compositions meeting specific needs and from which articles of specific physical properties may be molded. For example, there is a need for polycarbonate resins which may be molded into engineering structural components having certain thickness specifications, impact resistance and resistance to hydrocarbon solvent degradation. These articles are of particular need in the automotive industry as components of automobiles.
However, polycarbonate based molding compositions have had restrictions on their use in molding thick-walled articles due to two inherent disadvantages.
The first disadvantage is the low critical thickness values of polycarbonates, i.e., the thickness at which a discontinuity in Izod impact values occurs. These low critical thickness values tend to limit wall thickness of molded polycarbonate to a thickness below the critical thickness. Polycarbonates exhibit notched Izod impact values which are dependent on the thickness of the polycarbonates. Thus, for example, while typical notched Izod impact values for a 3.2 mm thick polycarbonate test specimen are generally in the range of about 850 J/M, typical notched Izod impact values for a 6.4 mm thick polycarbonate test specimen are generally in the range of about 160 J/M. The high Izod values of the 3.2 mm thick polycarbonate test specimen are due to the fact that these specimens are thinner than the critical thickness of the polymer and, therefore, upon impact a ductile break occurs. The low Izod impact values of the 6.4 mm thick polycarbonate test specimens are due to the fact that these specimens exceed the critical thickness of the polymer and, therefore, upon impact a clean or brittle break occurs.
It has been proposed that this thickness sensitivity can be reduced by adding a small amount of a polyolefin, e.g., polyethylene, to the polycarbonate resin. While the addition of polyethylene has proven to be somewhat effective, in the main thick section notched Izod strengths are still not comparable to thin section strengths.
The compositions of the present invention solve this particular problem of the prior art in that they are moldable into articles having a thickness both above and below the critical thickness with useful impact resistance properties.